Some of these problems directly
impact the audio and/or picture quality. Let's examine each type of powerline
disturbance below. Please understand that a full discussion of each of these
issues would require a large chapter in a book -- these definitions merely
scratch the surface.
In order to fix a problem,
it helps to define the problem. Some of the problems are electrical only,
but manifest themselves in the audio or video circuitry. Sometimes a problem
actually originates in the audio circuitry itself.
Some percentage, perhaps 5% to 25% above
of the "stated", "nominal", or "specified voltage".
|Undervoltage or Brownouts
Brownouts are the opposite
of the overvoltage condition, above. The original issue of brownouts was
when in large cities, heavy current demand for air conditioners in the summer
put such a strain on the power grid that the voltage sagged, or went down.
Sometimes this happens TO the power companies, and sometimes this is caused
BY the power companies. (i.e. controlled or rolling brownouts) The end result
is the same: less current is available and lower voltage is available.
Undervoltage condition caused
by the powerline source being too high an impedance to supply the necessary
current demand; could be milliseconds (such as from a motor start capacitor
drawing inrush current ) or many seconds (any continual appliance, such
as an electric iron, clothes dryer, heater, etc. This is more often than
not caused by too small a wire used in an installation or by Aluminum wire
used in place of copper.
The sine wave simply stops
for a few milliseconds, or perhaps as long as a few cycles. This might be
caused by a lightning protection changeover relay device somewhere upstream
in the power grid, or, it could even be caused by an intermittent connection,
and although that would be a rather rare condition, it does happen.
Higher voltage occurences
which ride on top of the sinewave; perhaps up to 750V or so. A spike might
be considered the shortest duration, perhaps a few microseconds to a few
milliseconds. A surge might be from a few milliseconds to a few seconds.
And a peak condition might last from a few seconds to a few minutes.
Perhaps 500 to 5000V lasting
from hundreds of nanoseconds to hundreds of microseconds. It is these higher
voltage spikes which burn out equipment, often in mysterious ways. Typically
these high voltage spikes are caused by lightning somewhere on the grid,
from a few feet to a few miles away
The opposite of peaks: V-shaped cutouts in
the sine wave as seen on an oscilloscope
Power Factor is the phase
relationship between the VOLTAGE condition of the line and the CURRENT condition
of the line. A totally resistive load (i.e. a lightbulb) has "unity"
power factor. A load that has a CAPACITIVE or INDUCTIVE component REACTS
with the sinewave; therefore this is said to be a "reactive" load.
This causes a harmonic imbalance of the sinewave, resulting in Harmonic
Distortion, explained below. One goal of load balancing for example on a
240 Volt circuit is to even out the different power factors so as to NOT
introduce either a voltage imbalance or a harmonic imbalance on the power
Odd and/or even order distortion
of the 60 Hz sinewave causes mechanical humming in transformers. The waveform,
instead of being a 'pure' 60 Hz sinewave, contains some combination of even
(120, 240, 360, 480 Hz) or odd (180, 300, 420, 540 Hz) harmonics.
|Saturation Noise (in
Another mechanical 'humming'
sound emanating from a transformer: the transformer windings are magnetically
saturated, often because the voltage is too high. This saturation starts
turning the sine waves into square waves. Square waves, by nature, are composed
of a train of odd harmonics. These harmonics mechanically vibrate the transformer
and its mounting apparatus, and that is what you hear 'mechanically'. In
the instance of a subwoofer, you might think the subwoofer [speaker] is
making the noise, but actually it's the transformer inside that is making
the noise and you are hearing this transformer noise through the
paper or cone material of the speaker. This noise is often a mix of both
60 Hz and 120 Hz, and some of both of their odd harmonics.
RF frequencies (radio frequencies,
i.e. from an AM radio station, TV station, CB radio, Ham/ Taxi / Police
/ industrial walkie-talkie radio, etc etc) amplitude modulating the sine
wave envelope. This is commonly referred to as "picking up RF".
See also rectification, below.
Noise common to BOTH sides of the line (sometimes
a problem when operating from 240 Volts)
Noise showing up on one side
of the line measured to ground. Therefore in the case of a 240 V line the
noise would be only or mostly on ONE side or leg, i.e one of the 120 V "legs".
Ground noise or currents
caused by bad, loose, or high impedance ground connections; can also be
caused by rectification in the grounding wiring.
Usually caused by dissimilar
metals such as aluminum / copper junctions which then rectify the RF in
the air and add the resulting modulation voltage to the power line.
Where some digital "audio"
or "power supply" circuit feeds its switching noise back into
the power line and the noise then shows up in another piece of equipment.
Sonic or ultrasonic "whine"
from a poorly designed switching power supply leaking back into the power
line. Expect a whole new generation of problems with so-called "switching"
audio amplifiers - especially as they get old.
Caused by loose or dirty
mechanical connection; even caused by such seemingly innocent devices as
pushing a wire into a captive wire slot in a switch instead of properly
screwing the wire down securely under the connection screw. May also come
from relays or motor brushes. If a switch was opened or closed at EXACTLY
the zero crossing point of a sine wave, since there is no voltage present
there is no current flowing. Since this is not a very likely occurrence,
most of the time when either a relay opens or a person opens a switch the
waveform is at some other spot than zero, and therefore there IS current
flowing. The current attempts to continue to flow across the airgap, causing
arcing. Arcing then oxidizes, pits, and eventually wears out the mating
surfaces. Some [better] circuits use relays filled with nitrogen which will
not allow an oxidizing arc to form... since there's no oxygen inside the
Often amplified noise from
the previous circuit; this often implies improper gain staging or matching,
but may be a defective component as well, acting as a noise 'generator'.
|Audio Noise: Hum at
A true ground loop! A true ground loop is
ONLY a 60 Hz component !
|Audio Noise: Hum at
Quite likely a power supply
filter problem; however most people "think" that 120 Hz hum is
a ground loop.
Perhaps a bad capacitor,
noisy transistor junction, or IC / OpAmp going bad or latching up.
Buzzing is typically 120
hz that is clipped, (or where the filtering circuitry in the power supply
has faulted) generating a series of ODD harmonics, i.e. a mix of 120, 360,
600, 840, 1080 Hz, distinctly measureable by specific test equipment, such
as a spectrum analyzer. Since it is clipped, it has turned into square waves;
square waves by definition are odd-order harmonics. So a true buzz has a
very recognizable sound. Please note that Hum and Buzz are completely different
|Audio Noise: Rectification
Hearing the demodulated audio
from AM, FM, TV, ham, shortwave, Marine, Taxi, CB, etc etc. This is NOT
the same as leakage, where one signal leaks into another undesired area
usually because solid state "switches" are used rather than mechanical
|Audio Noise: Common
A sound that seems ike a
combination of a buzz and noise, typically caused by "office"
fluorescent lights and their starter circuitry.
|Audio Noise: Transverse
This is the noise you hear
from "dimmers" where the buzzing changes pitch as the dimmer knob
is rotated; you are hearing the duty cycle of the dimmer change.
|Audio Noise: Ticks and
Any overvoltage condition
such as a spike will usually produce a tick in an audio circuit. Only the
very best power supplies will really filter this out, and even they have
no control of this if the tick is riding on an audio signal which is properly
going through an amplifier. This could also be caused by a refrigerator
motor or washing machine motor (on the same circuit) starting up. The higher
impedance the entire chain of wiring is, the more likely this will happen
and be noticed.
|Audio Noise: Video Interval
59.94 hz beating against
60 Hz; often caused by ground differentials between a feed from a "TV
cable company" and the local house or studio ground; perhaps as high
as 65 volts differential. This manifests itself as a buzzing sound, cyclical
in nature, sort of repeating every 14 seconds. Once this sound is heard
and memorized it is VERY recognizable.
There are two series of superb and rather
A series of articles by Bill Whitlock, here: www.jensen-transformers.com/apps_wp.html